Current-limiting fuses comprising fuse links of silver and copper



CURRENT-LIMIT'INGFUSES COMPRISING FUSE LINKS OF SILVER AND COPPER Feb.12, 1957 K w SWAIN 2,781,434

Filed Jan. 6, 1955 2 Sheets-Sheet 1 1957 K. w. SWAIN 2,781,434CURRENT-LIMITING FUSES COMPRISING FUSE LINKS OF SILVER AND COPPER FiledJan. 6, 1955 2 Sheets-Sheet 2 5 All Wily/1111111 74 \v United StatesPatent CURRENT-LHVIITING FUSES COMPRISING FUSE LINKS OF SILVER ANDCOPPER Kenneth W. Swain, Hampton Falls, N. H., assignor to TheChase-Shawmut Company, Newburyport, Mass.

Application January 6, 1955, Serial No. 480,080

7 Claims. (Cl. 200-131) This invention relates to the protection ofelectric circuits, electric apparatus and electric machinery by electricfuses.

It is one of the objects of this invention to provide relativelyinexpensive current-limiting fuses highly sensitive to major faultcurrents comprising composite fuse links of silver and copper.

Another object of the invention is to provide currentlimiting fusescomprising composite silver-copper fuse links wherein the amount ofsilver is minimized and which are nevertheless capable of limiting majorfault currents at least to the same extent as comparable fuses havinglinks consisting entirely of silver.

Still another object of the invention is to provide current-limitinghigh current-carrying capacity fuses comprising ribbon-type fuse linksincluding a strip of silver and a pair of strips of copper bondedtogether to form a virtually unitary strip of sheet metal.

Further objects and advantages of the invention will become moreapparent as the following description proceeds, and the features of'novelty which characterize the invention will be pointed out withparticularity in the claims annexed to, and forming part of, thisspecification.

For a better understanding of the invention reference may be had to theaccompanying drawing wherein Fig. 1 is a side elevation of the materialused for manufacturing fuse links of the type used in fuses embodying myinvention;

Fig. 2 is a section along 2-2 of Fig. 1;

Fig. 3 is a side elevation of a fuse embodying my invention;

Fig. 4 is substantially a longitudinal section through the fuse shown inFig. 3 along 44 of Fig. 3;

Fig. 5 is a front elevation of a stamping to be used as fuse link for ahigh current-carrying capacity fuse em bodying my invention;

Fig. 6 is a section along 6-6 of Fig. 5;

Fig. 7 is a front view of a stamping which may be used to form acylindrical fuse link;

Fig. 8 is substantially a longitudinal section through a fuse comprisinga fuse link made of a stamping of the type shown in Fig. 7, and

Fig. 9 is a section along 9-9 of Fig. 8.

The material shown in Figs. 1 and 2 comprises a strip of silver 2 andtwo spaced strips of copper 1 each arranged on opposite sides of thestrip of silver 2. Strips 2 and 1 are bonded together to form asubstantially unitary strip of sheet metal. The processes for producingsuch composite metals are well known in the art, and therefore do notneed to be described here in detail. The width of each strip of copper 1is considerably larger than the width of 'the intermediate strip ofsilver 2. This tends to minimize the cost of the material required formanufacturing fuse links for fuses embodying my invention. Adash-and-dot line D in Fig. 1 indicates the way in which stampings maybe made from the strip ICC therewith. Fuse link 3 extends in a directionlongitudinally of the fuse and interconnects conductively the twoferrules or terminal caps 5. The two ferrules 5 are mounted on a tubularcasing 4 of an appropriate insulating material such as, for instance, aglass-clothsynthetic-resin laminate. Each of the ferrules 5 is providedwith a slot 5a for insertion of one end of fuse link 3. The axiallyouter ends of fuse link 3 which enter slots 5a may be slightly reducedin width compared to what may be called the normal width of link 3. Sucha reduction in width may be desirable for manufacturing reasons but hasno effect upon the performance of the fuse. Each slot 5a in each cap orferrule 5 is substantially closed by a washer 6 to preclude thesilicious pulverulent arc-quenching filler 7 within casing 4 fromflowing through the slots 5a in ferrules 5 out of casing 4. Each ferrule5 is soldered to one of the axially outer ends of fuse link 3 tominimize the resistance of the current-path formed by the fuse. Anaxially outer cap or ferrule 8 is mounted under pressure on each of theaxially inner caps or ferrules 5, and both the axially outer and theaxially inner caps or ferrules are crimped at their axially inner endsto produce a firm and gas-tight connection with casing 4. Each of theaxially outer caps or ferrules 8 is provided with a blade contact 9extending away from the casing 4 of the fuse.

The strip 2 of silver of which fuse link 3 is made is arranged in thecenter region of easing 4, i. e. midway between the ends thereof. Silverstrip 2 has a point of reduced cross-sectional area 2a coextensive withthe center of casing 4 and with the hot spot formed on said link 3 whencarrying an electric current. Each of the strips of copper 1 of whichlink 3 is made begins at opposite axially outer boundary lines 2b ofsilver strip 2, and each strip of copper 1 provides a direct currentpathfrom the silver strip 2 to one of the terminal elements 5, 8. The pointof minimum cross-sectional area 2a is situated exactly in the middlebetween boundary lines 2b. Each copper strip 1 is considerably longerthan the intermediate silver strip 2, i. e. the length of each copperstrip 1 is a multiple of the length of silver strip 2. The length ofcopper strips I ought to be at least twice the length of silver strip 2.

The above referred-to point of reduced cross-sectional area 2a of silverstrip 2 is produced by two lateral substantially V-shaped notches 20substantially coextensive with the center of, and the hot-spot formedon, link 3 when the latter is carrying current. The edges of notches 2cenclose an acute angle. The length of the silver strip 2 is in the orderof the width of notches 20 at the wide open end thereof. The depth ofeach notch 20 is almost equal to one half of the width of strip ofsilver 2 where its width is greatest; consequently the cross-sectionalarea of the point of reduced cross-sectional area 2a formed by notches2c is very small. The length of the portion of minimum cross-sectionalarea 2a of silver strip 2 is also very small. As a result of thisconfiguration of silver strip 2, its point of reduced cross-sectionalarea 2a approximates closely a point heat source when link I 3 iscarrying an electric current. It will be apparent from the foregoingthat in the fuse illustrated and described the point of the fuse linkwhere the maximum heat is generated is arranged at a point if heatgeneration is substantially localized at the center of. the casing.Itis, therefore, possible to producefuses having a relatively highoperating temperature at the center of the casing and of the link, whichnevertheless run. cool on account of small I -r losses required toachieve the high hot-point temperature on the link. Since the neck orreduced cross-section portion 2a of the fuse link. operates normally ata high temperature and since it is made of the low fusing. energy metalsilver but. little additional energy is required subsequent to faultinception to heat the neck 20 of the link from its normally highoperating temperature to fusing temperature. The fuse is therefore veryfast, i. e. tends to limit the peak of major fault currents way belowthe peak of. theravailable short-circuit currents. This is achieved inspite of the fact that theprepondcrant portion of the length of the linkconsists of copper which has a relatively high fusing energy.

In this fuse structure silver is used to initiate the are as early afterfault inception as possible to reduce the peak of the let-throughcurrent to as low a level as possible. Once arcing is initiated and thecurrent-limiting action of insertion of arc resistance into the circuitstarted, the terminals of the arc may be safely transferred byback-burning from the axially inner small silver portion of the link tothe relatively long portion thereof which consists of copper. As far ascurrent-limiting action is concerned, the link functions in the same wayas a well pre-heated all-silver link. As far as all other phases of theoperation of the link are concerned, including its current-carryingability, the link behaves, in essence, in the same way as an all-copperlink. Because of the small quantity of silver involved, the cost of thelink is almost as low as that of an all-copper link, though thecurrent-limiting action thereof is equal to that of an all-silver linkdesigned for drastic currentlimiting action.

The silver strip 2 and the narrow neck 2a formed therein operate assupersensitive arc-kindling means, i. e. they kindle an arc and initiatethe process of insertion of arc resistance at an early time followingfault inception. The silver strip 2 does not need to be as long as therequired ave-rage back-burning distance of the link, i; e. theback-burning distance of the link in the given surrounding medium and ina given circuit under conditions of average severity as determined bythe fault angle, etc. For reasons of economy the length of the silverstrip 2 ought to be less than the average backburning distance of thelink 3. The two strips of copper 1 forming direct axial extensions ofthe strip of silver 2 are adapted to support additional back-burningupon complete vaporization of silver strip 2 under the heat of the arcformed upon blowing of the fuse. Where the severity of interruption issmall, the current-flow through the circuit protected by the fuse maycease before all of the silver strip 2 is heated to vaporizationtemperature and vaporized.

It will be apparent from the foregoing that the strip of silver 2decreases progressively in width from a point of largest width to apoint of minimum width and increases progressively in width from saidpoint of minimum width to a point of largest width. Fuse links withgradual change of cross-sectional area compare favorably with fuse linkswherein changes in cross-sectional area are abrupt.

In some instances it is necessary or desirable to provide for fuseprotection not only in cases of major faults but also in cases of smallprotracted overloads. To this end an element of a link-destroying lowfusing point metal may be arranged on the link 3 at a point thereofcloser to the point of reduced or minimum cross-sectional area 2a thanto the axially outer boundary lines 212 of silver strip 2. In Fig. 4 arivet 10 of tin, or indium, or of a tin-indium alloy has been shown asthe preferred a form of adding a link-destroying low fusing point ele'mentto fuse link 3.

Because of the fact that element 10 is arranged close to the neck orpoint of reduced cross-section 2a of silver strip 2, the temperature ofelement 10 will always be relatively high, though always lower than thatat the neck or point of reduced cross-section 2a. Consequently, the neck2a tends to be at a higher temperature than element 10 when element 10reaches its fusing temperature on account of a protracted smalloverload. Upon fusion element 10 tends to flow to the immediatelyadjacent point of reduced cross-section where the prevailing temperatureis higher. There a very rapid metallurgical reaction between the lowfusing point metal and the silver takes place. This reaction results inthe formation of an alloy which has a considerably higher resisitivitythan silver, which in turn is conducive to increased generation of heatat the neck portion 2a, and consequently in linkdestruction at anaccelerated rate.

If element ,10 is made of induim, or a tin-indium alloy, relativelylonger delay times are achieved than if the element 10 is made. of tin,or any other low fusing point metal having the required link-destroyingproperties. Indium alloys readily with silver at the fusing point of theformer but does hardly alloy with copper; hence the desirability ofproviding a fuse link which consists prepond'erantly of copper with asmall section of silver performing the function of imparting to thelinkthe intense current-limiting action of an all-silver link, andincreasing the range of time lags by allowing the use of indium, orindium alloys, as link-destroying, or derating, agents.

Referring now to Figs. 7 to 9, reference numeral 1' has been applied toindicate a pair of spaced strips of copper and reference numeral 2 toindicate an intermediate silver strip. Strips 2' and 1' are bonded.together to form a virtually unitary strip of sheet metal. Silver strip2 is provided with a line of diamond-shaped perfo rations 20- which takethe place of the V-shapcd notches in the embodiment of the inventionshown in Figs. 3 and 4. A very narrow current-path is produced betweeneach pair of immediately adjacent diamond-shaped perforations, resultingin a line of high spot-temperatures when a link made up of stamping 1',2' is carrying current.

Fig. 8 shows a fuse comprising a fuse link 3 made up of a stamping ofthe kind shown in Fig. 7 rolled to form a substantially tubularconductor of current. The fuse structure shown in Fig. 8, except forthat of the link, is more fully disclosed in the copending patentapplication of Frederick J. Kozacka, Ser. No. 436,398, filed June 14,1954, for Power Fuses with Tubular Links and Pressure-Type LinkConnections, now U. S. Patent 2,777,033 and reference is made to thispatent for a -more complete disclosure of this structure.

The structure shown in Fig. 8 comprises, in essence, a pair of coaxialcylindrical copper blocks 11 arranged in spaced relation andconductively interconnected by tubular fuse link 3. Each copper block 11is provided at the axially inner end thereof with a cylindricalprojection 11' on which link 3 is mounted. Split rings 12 are mounted onboth ends of link 3 and pressed firmly by the outer rings 13 against theprojections 11' of blocks 11. Blocks 11 are held in spaced relation bycasing 14 pinned to blocks 11 by transverse steel pins 15. Each block 11is provided with a blade contact 16 for insertion of the fuse into afuse holder, or the like. The cylindrical space within tubular link 3'and the toroidal space between link 3 and casing 14 may be filled with apulverulent arcquenching filler 17. To indicate the two alternatives, i.e. presence or absence of filler 17, a portion of Figs. 8 and 9 has beenshown with the filler added, and another portion with the filleromitted.

Although this invention has been described in considerable detail, it isto be understood that such description is intended illustrative ratherthan limiting, as the invention may be variously embodied, e. g. inmulti link rather than single link fuses, and is to be interpreted asclaimed.

I claim:

1. In a current-limiting fuse the combination of a substantially tubularcasing, a pair of terminal elements each arranged on one end of saidcasing, a ribbon-type fuse link comprising a strip of silver and a pairof strips of copper bonded together to form a virtually unitary strip ofsheet metal, said strip of silver being arranged in the center region ofsaid casing and having a pair of lateral substantially V-shaped notchescoextensive with said center region and the hot-spot region on said linkwhen carrying current, the width of the wide open end of each of saidpair of notches being substantially equal to the length of said strip ofsilver, the ed es of each of said pair of notches enclosing an acuteangle, and said pair of notches defining a short length of reducedcross-sectional area on said strip of silver closely approximating apoint-heatsource, each of said pair of strips of copper beginning atopposite axially outer boundary lines of said strip of silver and eachproviding a direct current-path from said strip of silver to one of saidpair of terminal elements.

2. In a current-limiting fuse the combination of a substantially tubularcasing; a silicious pulverulent arcquenching filler Within said casing;a pair of terminal elements each arranged on one end of said casing, aribbontype fuse link comprising a strip of silver and a pair of stripsof copper bonded together to form a virtually unitary strip of sheetmetal submersed in said filler, said strip of silver being arranged inthe center region of said casing and having a pair of lateralsub-stantially V-shaped notches, the edges of each of said pair ofnotches enclosing an acuate angle and said pair of notches defining ashort length of reduced cross-sectional area on said strip of silverapproximating a point-heat source coextensive with said center region ofsaid casing and the hot-spot region on said link when carrying anelectric current, the length of said strip of silver being in the orderof the width of said pair of notches at the wide open end thereof, andeach of said pair of strips of copper beginning at opposite axiallyouter boundary lines of said strip of silver and each providing a directcurrent-path from said strip of silver-to one of said pair of terminalelements.

in a fuse the combination of a substantially tubular casing ofinsulating material, a pair of terminal elements each arranged on oneend of said casing, a ribbon-type fuse link comprising a strip of silverand a pair of strips of copper bonded together to form a virtuallyunitary strip of sheet metal, said strip of silver being arranged in thecenter region of said casing and having a pair of lateral substantiallyV-shaped notches defining a short length of reduced cross-sectional areaclosely approximating a pointheat-source when said link is carrying anelectric current, said short length of reduced cross-sectional areabeing coextensive with the center of said casing and the hotspot on saidlink when said link is carrying an electric current, each of said pairof strips of copper beginning at opposite axially outer boundary linesof said strip of silver and each providing a direct current-path fromsaid strip of silver to one of said pair of terminal elements, and arivet of a link-destroying low fusing point metal on said strip ofsilver at a point thereof closer to said short length of reducedcross-sectional area than to said axially outer boundary lines.

4. In a current limiting fuse the combination of a substantially tubularcasing; a silicious pulverulent arcquenching filler within said casing;a pair of terminal elements each arranged on one end of said casing, aribbontype fuse link comprising a strip of silver and a pair of stripsof copper bonded together to form a virtually unitary strip of sheetmetal submersed in said filler, said strip of silver comprising aportion of reduced crosssectional area arranged in the center of saidcasing, the length of said strip of silver being less than the averageback-burning distance of said link, each of said pair of strips ofcopper beginning at opposite axially outer boundary lines of said stripof silver and providing a direct current-path from said strip of silverto one of said pair of terminal elements, and the length of each of saidpair of strips of copper exceeding the length of said strip of silver.

5. In a current-limiting fuse the combination of a substantially tubularcasing, a pair of terminal elements each arranged on one end of saidcasing, a ribbon-type fuse link comprising a strip of silver and a pairof strips of copper bonded together to form a virtually unitary strip ofsheet metal, said strip of silver being arranged in the center region ofsaid casing and having a point of reduced cross-sectional area midwaybetween the opposite axially outer boundary lines thereof, each of saidpair of strips of copper beginning at said opposite axially outerboundary lines and providing a direct current-path from said strip ofsilver to one of said pair of terminal elements, and the length of saidstrip of silver being less than the average back-burning distance ofsaid link.

6. In a current-limiting fuse the combination of a substantially tubularcasing; a silicious pulverulen-t arcquenching filler in said casing; apair of terminal elements each arranged on one end of said casing, aribbon-type fuse link submersed in said filler, said link comprising astrip of silver and a pair of strips of copper bonded together to form avirtually unitary strip of sheet metal, said strip of silver beingarranged in the center region of said casing and having a pair oflateral substantially V-shaped notches each comprising a pair of edgesenclosing an acute angle and defining a region of reducedcross-sectional area of said link, each of said pair of strips of copperbeginning at an opposite axially outer boundary line of said strip ofsilver and each providing a direct current-path from said strip ofsilver to one of said pair of terminal elements, the length of saidstrip of silver being less than the average back-burning distance ofsaid link and the length of each of said pair of strips of copper beinga multiple of said length of said trip of silver.

7. A current-limiting fuse comprising a substantially tubular casing; asilicious pulverulent arc-quenching filler within said casing; a pair ofterminal elements each arranged at one end of said casing, a fuse linkmade of an axially inner strip of silver and of a pair of axially outerstrips of copper bonded together to form a virtually unitary strip ofsheet metal submersed in said filler and conductively interconnectingsaid pair of terminal elements, said strip of silver having a portion ofreduced cross-sectional area arranged in the center of said casing tocause arc-initiation thereat, said strip of silver being shorter inlength than the required average back-burning distance of said link, andsaid pair of strips of copper forming direct axial extensions of saidstrip of silver adapted to support additional bacbburning upon completevaporization of said strip of silver under the heat of the are formedupon blowing of the fuse.

References Cited in the file of this patent UNITED STATES PATENTS348,048 Perkins Aug. 24, 1886 713,831 Badeau Nov. 18, 1902 1,215,999Schweitzer Feb. 13, 1917 2,703,352 Kozacka Mar. 1, 1955 FOREIGN PATENTS349,519 Great Britain May 27, 1931 468,916 Great Britain July 15, 1937

